When two or more substances are present in a liquid state or in a slurry state (hereinafter referred to as the liquid mixture), there have been many chemical methods for separating a particular content from the other contents. However, the conventional methods are not effective to separate one particular content from others when the contents are eutectic or form a solid solution, where the contents are so similar in chemical and physical properties that they are difficult to separate. As a result the common practice is to effect the separation of contents at particular temperatures, which will be referred to as cooling crystallization. This includes a cooling method under which a particular content is separated at its freezing point from the others.
However cooling crystallization has some disadvantages; for example, (1) it is difficult to control the temperature, (2) temperature gradients are likely to occur in the system, thereby preventing the achievement of thermal equilibrium and (3) the operational time is prolonged.
Therefore the inventors have started to solve the problems encountered in cooling crystallization and have developed a crystallization process utilizing pressure instead of heat, hereinafter referred to as pressure crystallization equipment.
As the pressure crystallization method is operated for commercial purposes, particularly using large-scale equipment, new problems have arisen. One of them is how to discharge the liquid phase out of the system under a high pressure when the liquid and solid phases co-exist as a mixture therein.
Under the pressure crystallization method with the rise in pressure the crystallization proceeds, whereas as the pressure is decreased the crystals melt or become soft. As equipment a cylindrical highpressure vessel is used, which is provided with a filter on its inside wall. Behind the filter the system is communicated with the atmosphere wherein the communication is blocked by means of a valve when it is intended to allow liquid and solid phases to co-exist. There are many modifications to it.
In operation a mixture in a liquid state is put in the vessel; the communication with the atmosphere is blocked and the discharge of a liquid is closed, which means that the vessel is completely closed. A high pressure is applied to the mixture in the vessel; if necessary, the temperature is reduced. In this way the particular content is crystallized in the mixture, thereby producing a state in which the particular fraction of crystals and the remaining liquid co-exist. Then the valve is opened so as to withdraw the liquid content, which is forced out through the filter by applying pressure to the mixture in the vessel. The pressure is continuously applied to the remaining solid phase so as to squeeze it and force the remaining liquid through the filter. In this way a highly pure content remains in the vessel.
Reference will be made to FIG. 1, which shows an example of a conventional high-pressure vessel. The vessel 1 has as filter 2, a thermal insulating material 3, a piston 4, a bottom closure 5, a mixture supply pipe 6, and a discharge pipe 7.
The steps taken to operate the illustrated vessel are as follows:
(1) A valve V7 is closed and the valve V6 is opened so as to allow the mixture to enter the vessel 1; PA0 (2) After the supply of mixture is completed the valve V6 is closed, and the piston 4 is caused to descend, thereby causing an increased pressure upon the mixture in the vessel. In this way the crystallization of a particular content is promoted; PA0 (3) After the crystallization is finished the valve V7 is opened and the subsequent filtering and squeezing start; that is, first, the liquid content in the vessel is squeezed, and caused to pass through the filter. The fluid content passes through a path 8, and is discharged through the pipe 7 via the valve V7; and PA0 (4) After the filtering and squeezing are finished, the vessel 1 is opened, the solid cake at the atmospheric pressure is taken out or alternatively, is melting for recovery.
In the process (3) mentioned above the filter 2 is subjected to as high pressures as 500 atms or more, sometimes a few thousands atms, in the direction of arrow (A), that is, perpendicular to the filter surface. In addition, as the squeezing advances, the filter surface is subjected to a frictional force involved in squeezing the solid cake wherein the frictional force acts on the filter surface in the dirction of arrow (B), that is, in the axial direction. Furthermore, a differential pressure between the high internal pressure acting on the upper ring 2a and a possible atmospheric pressure thereunder affects the filter surface. Owing to these combined factors the filter is in danger of compressible deformation in the direction of arrow (B), and sometimes in danger of expanding deformation in the direction of arrow (A); sometimes, the filter breaks owing to the expanding deformation. When the filter is made of a sintered metal (SUS or the like) having a simple structure it often happens that the pores in the filter are crushed and clogged, thereby losing the filtering ability.
As described above the conventional pressure segregation method has a great disadvantage of filter fracture and/or lost filtering ability. This leads to a reduced efficiency in the form of low yields or the reduced purity of a collected content.
In order to solve these problems occurring in the process of filtering and squeezing it is essential to develop a cylindrical filter strengthened radially as well as axially.
In order to overcome the difficulty mentioned above the inventors have made an invention for which a patent application No. 59(1984)-50108 has been filed. In this invention the upper portion of the inside wall of the filter is fixed to the ring, and the filter is backed up by a reinforcement and fixed at a given place in the vessel so as to protect the filter against axial deformation.
As is shown in FIG. 2 the entire structure is substantially the same as the embodiment of FIG. 1. A ring 9 is fitted in a space above the filter 2 and a cylindrical reinforcement 10 is provided behind the filter 2 through a thermal insulating material 2. The reinforcement 10 is so constructed that its length can be adjusted in accordance with the axial length of the filter so as to locate the upper ring 9 at a desired position. The reinforcement 10 can be cylindrical when the insulating material 3 is interposed against the filter 2 or vertically split for facility of attachment and detachment. The insulating material 3 has a structure which permits the liquid to pass therethrough toward the discharge path 8. When no insulating material 3 is interposed, it is necessary to provide a vertical slit whereby the liquid is permitted to flow after having passed through the filter 2.
The size of the slit must be determined so as to reinforce the filter structure in the axial and radial directions. It is possible to make the ring 9 in one body with the reinforcement 10.
In this way the filter 2 is fixed at a particular place with the ring 9 and reinforcement 10, and if it is additionally backed up by the reinforcement 10 the filter 2 is protected against deformation in the directions of arrows (A) and (B) and against its pores being crushed or expanded. Thus the high yield and the purity of a collected component are ensured. Because the filter 2 is protected against becoming damaged the frequency of replacing the filter with a new one is considerably reduced.
FIG. 3 shows another embodiment of the previous invention, which is characterized in that the filter 2 is fixed at its upper and lower ends by means of rings 9 with the reinforcement 10 interposed therebetween. Owing to the rings provided at both ends the filter 2 is safely protected against compressive and expanding deformation.
FIG. 4 shows a further example of the embodiment. In this example the cylindrical filter 2 has a progressively divergent wall, that is, a tapered wall. The reinforcement 10 and the insulating material 3 are equally shaped, and they are provided between the upper and lower rings 9, with a spacer 11 being provided in the outermost layer. The spacer 11 is cotter-shaped in cross-section as shown in FIG. 4.
In order to prevent the filter from becoming deformed along its circumference it is necessary to minimize the space between the insulating material 3 and the reinforcement 10 located behind the filter 2. In the examples shown in FIGS. 1, 2 and 3 where the filter 2 is purely cylindrical, the minimized space is almost impossible when the facility of inserting the filter is taken into consideration. The embodiment shown in FIG. 4 has solved this difficulty. Spacer 11 functions as a wedge, thereby placing the reinforcement 10 into tight contact with the back of the filter 2 to the extent that no space is produced therebetween. In addition the filter is prevented from becoming deformed in a radial direction. When the filter 2 is to be removed the block 5 has only to be pushed downward. In the process of squeezing, the filter 2 is subjected to a downward frictional force. As shown in FIG. 5 (the straight line shows a pre-squeezing state and the broken line shows an under-squeezing state) the solid cake is subjected to a force whereby it is urged to separate from the inner surface of the filter 2, which means that the friction lessens. The solid cake is squeezed when it is in the state shown by the broken line in FIG. 5 as it is subjected to a series of buckling fractures during which a liquid path is produced inside the cake, thereby facilitating the separation of liquid and solid.
In the embodiments mentioned above the filter is made of sintered metal mesh, but the material is not limited thereto: mono- or multi-layer metal mesh, a porous plate, a laminated sintered material, a piece of canvas or their combination can be selected in accordance with the pressure and the nature of the treating material.
The results of the research conducted prior to the present invention have been described as the background of the present invention. The contents of the research has not yet been published and is not available to the public. Under the new pressure crystallization method the filter sometimes has fractured and a clogging trouble has occurred. There has been a strong demand for an improved pressure segregation method and equipment.
The next problem is how to control the temperatures during the operation.
The pressure segregation method uses pressures as a variable, but heat is unavoidably generated and it is necessary to control the temperatures so as to minimize the influence of heat. In order to carry out the pressure crystallization method efficiently it is essential to control the temperatures likely to rise in the course of operation. One solution is to place the mixture kept at lower temperatures in the vessel. However in the processes (2) and (3) heat is likely to generate and the temperature in the vessel instantaneously rises by 10 degrees or more. The wall of the vessel has such a large heat capacity that the heat generated is absorbed in the wall, thereby restraining a further rise in the temperature. As the heat radiates, the temperature lowers. As a result there arises temperature gradient in the vessel, which results in uneven segregation. In such situations the collecting liquid content is likely to crystallize in large quantity on and around the filter and/or in the discharge path, thereby preventing the smooth separation of liquid and solid contents. There is another solution, that is, the pressure in the vessel is slightly reduced so as to melt the crystals of lower purity, and enhance the purity of the collected solid content. When the pressure is reduced in this way the treated substances tend to have lower temperatures than the filter and the inside wall of the vessel, which may lead to the excessive melt of solid phase to change it.
The temperature gradient is an important problem in that (1) it is likely to impair the purity of a collecting content because of a possible crystallization of other than the desired content, (2) the discharge path is likely to become clogged, thereby reducing the performance of the filter, and (3) the crystals of the particular content are likely to melt, thereby resulting in a reduced yield.
The above-mentioned problems are amplified by daily and seasonal changes in temperature which affect the temperature of the vessel, thereby requiring a strict control of the temperatures of the vessel.
However the control of temperature is very complicated, and is a labor- and money-consuming work. In addition the vessel unavoidably has a large heat capacity because of its construction of thick metal. This makes the control of temperature difficult. One solution is that the vessel is constructed such that heat transfer between the wall of the vessel and the inside thereof is minimized. The embodiments shown in FIGS. 1 to 4 have achieved this solution to some extent.
Another difficulty is how to take out solid cakes remaining in the vessel after the filtering and squeezing have been finished. When the vessel is purely cylindrical having an equal diameter along its entire length, a high pressure such as 500 atms or more than 1,000 atms acts on the inside of the filter. Such a high pressure can break not only the inside of the filter but also expand the wall of the vessel. Under this situation the particular content to be collected is tightly packed in a solid cake in the vessel after the squeezing has been finished. At this stage the piston is raised so as to release the high pressure and restore a normal atmospheric pressure in the vessel. Then the piston is again lowered, or the vessel is lifted up against the piston, so as to push the solid cake. However when the pressure is reduced to the normal pressure as mentioned above, the wall of the vessel tends to restore its original capacity. As a result the solid cake is tightened along its circumference, thereby placing it into tight contact with the inside surface of the filter. In this situation if the solid cake is too strongly pushed by the piston toward the bottom of the vessel a large friction acts in between the solid cake and the filter, thereby damaging the inside surface of the filter. Thus the life of the filter is shortened. Frequent replacement will be necessary. To solve this problem one way is to use a thick filter, which, however, leads to a high cost. In this respect the embodiment of FIG. 4 has been found effective.
There is a further problem encountered in the separation of a liquid content from the liquid/solid mixture:
The liquid content is kept at a high pressure, and when it is discharged to a low-pressure side through the filter and the discharge pipe, there arise a sudden drop in pressure, which transfers to the solid phase in the vessel. As a result the solid phase begins to melt, thereby reducing the working efficiency. Therefore it is necessary to keep the high pressure in the vessel as extensive as possible. One basic idea is to provide a pressure buffering chamber such as a hatch. On the basic of this idea there are (1) a method of using a pump connected to the high-pressure side, whereby the high-pressure liquid is pumped as it is at the high pressure from the solid phase, and then it is withdrawn to the low-pressure side, or alternatively, (2) a method of placing a anti-pressure device of the same type and size as the pressure segregation equipment at a place adjacent thereto. As evident from the description these methods are disadvantageous in being costly and complicated.
At any rate it is difficult to take out a high-pressure liquid content to a low-pressure side while the liquid is being kept at the high pressure, and it is essential to develop an equipment achieving this difficult task.
In association with the above-mentioned difficulty there is a problem of clogging in the discharge path owing to the high-pressure-unavoidably acting on the discharge path thereby to produce crystals under the influence of the pressure. When the clogging occurs in the discharge path is is necessary to melt and remove the crystals by an extra step. This is also a time- and labor-consuming work. The regular course from supply of material to discharge of the product takes a few minutes, and in normal operation the process is continously repeated day and night. If the discharge path clogs, the regular course of operation is broken. The working efficiency is considerably reduced. In order to solve this problem one solution is to heat the entire equipment at a certain temperature. This requires a lot of heat energy, which reflects in the high production cost. In addition if heat is added to the total equipment it must be covered with an insulating material so as to keep the heat, thereby making it difficult to observe from the outside. For example it is difficult to inspect a liquid leak in the couplings or joints in the equipment.
As has been so far pointed out the conventional pressure crystallization method and equipment have many problems arising after the crystallization has taken place, and the present invention aims at solving them and increasing the efficiency of separating a particular content from the remaining liquid mixture.